We studied nutrition requirements of small colony mutants ofStaph. aureus, that are resistant to 10, 000γ/ml of SM, and also the resistance against SM, of large colony strain arising out of reverse mutation, and obtained the following results.1. When cultured in the media containing hemoglobin, glutamic acid, glucose, glycerin, filtrate of live bacteria in culture, and filtrate of dead bacteria (killed by heating), there could be observed no accelerating action on the cell growth of these strains.2. When cultured in the common agar medium containing 1% hemoglobin, there arose large colonies by reverse mutation, and likewise when cultured in the common agar containing the filtrate of live bacteria in culture, there developed many large colony mutants. In the case cultured in common agar plate medium there could be recognized a few large colony mutants.3. We also attempted to restore the SM resistance of large colony mutant, namely, reverse-mutant strain, but could not recognize any change in the SM resistance.